Thrombosis refers to thrombus formation inside blood vessels, subsequent to a combination of hereditary and acquired risk factors, known as thrombophilia or hypercoagulable states. Vessel wall damage, stasis, increased platelets reactivity and activation of clotting factors are some of the fundamental features of thrombosis. Thrombosis can occur in both venous and arterial circulation and can result in the development of deep vein thrombosis (DVT), pulmonary embolism, and stroke. If a thrombus occurs in the arterial system, down-stream ischemia can occur, leading to acute coronary syndromes (ACS), ischemic stroke, and acute limb ischemia. Thrombus formation in the venous system typically leads to deep venous thrombosis, pulmonary embolism and chronic thromboembolic pulmonary hypertension. Clots may also form in the left atrial appendage in patients with atrial fibrillation (AF), and dislodged thrombi may result in potentially devastating complications, i.e. thromboembolic stroke and systemic embolism. The currently available antithrombotic medications, including low molecular weight heparin (LMWH), thrombin inhibitors, and Factor Xa (FXa) inhibitors, are all associated with a significant risk of bleeding (Weitz J. I. (2010) Thromb. Haemost. 103, 62). The development of an antithrombotic agent that does not affect hemostasis, and therefore does not result in bleeding complications, as well as specific reversal agents, would be highly desirable.
Current anticoagulants are either injected or taken orally. The injectable anticoagulant LMWH is widely used and offers an improved therapeutic profile over formerly applied unfractionated heparin. For the past few decades the most commonly used oral anticoagulant has been warfarin. Warfarin has a narrow therapeutic window that requires frequent monitoring of the coagulation status, and shows a variety of drug-drug interactions. More recently, orally available direct FXa and thrombin inhibitors entered the anticoagulant market and are increasingly applied.
LMWHs, FXa inhibitors, and thrombin inhibitors are all efficacious in the prevention of post-operative venous thromboembolic disease, in the treatment of spontaneous DVT and pulmonary embolism, and in the stroke prevention in atrial fibrillation. However, these anticoagulants are also associated with bleeding complications that were generally comparable to those observed with the older drugs warfarin and unfractionated heparin. In the ADVANCE-2 clinical trial, the FXa inhibitor apixaban (Eliquis) was compared to the LMWH enoxaparin in patients after total knee replacement. While acute apixaban therapy was more effective at preventing venous thromboembolic disease than enoxaparin, both agents were associated with a significant risk of bleeding. Clinically relevant bleeding occurred in 4% of patients receiving apixaban and in 5% of patients treated with enoxaparin (Lassen, M. R., et al. (2009) N. Engl. J. Med. 361, 594).
In the RE-LY trial, the direct thrombin inhibitor dabigatran (Pradaxa) was compared to warfarin in patients with atrial fibrillation and a risk of stroke (Connolly, S. J., et al. (2009) N. Engl. J. Med. 361, 1139). Chronic dabigatran therapy was associated with a significantly lower risk of stroke or systemic embolism. However, major bleeding complications occurred in 3.1% of patients receiving 150 mg per day of dabigatran and in 3.4% of patients receiving warfarin (p=0.31).
Atrial fibrillation (AF) remains the most common cardiac arrhythmia in clinical practice, accounting for approximately one third of hospitalizations for cardiac dysrhythmias. Currently, it is estimated to affect more than 6 million patients in Europe and approximately 2.3 million in the United States, and this number continues to grow rapidly because of the increasing proportion of the aging population. It is estimated that approximately 5% of the population over the age of 65 years, and 10% of people aged over 80 years, will develop AF, however, the prevalence of AF is increasing beyond what is explained by age alone. AF risk factors such as hypertension, congestive heart failure, left ventricular hypertrophy, coronary artery disease and diabetes mellitus, and obstructive sleep apnea are also on the rise. As such, the number of affected individuals with AF is expected to increase two to three times over the next three decades in western populations. (Kannel and Benjamin (2008) Med Clin North Am. 2008; 92:17-40; Bunch, et al. (2012) J Innovations of Card Rhythm Manag 2012; 3: 855-63).
The principal risk of AF is a four- to five fold increase in embolic stroke. The attributable risk for stroke associated with AF increases steeply with age to 23.5% at ages 80 to 89. AF is associated with a doubling of mortality in both genders (Kannel and Benjamin 2008). AF is also independently associated with cognitive decline and all forms of dementia (Marzona, et al. (2012) CMAJ 2012; 184: 329-36; Geita et al 2013; Bunch et al 2012).
Most patients with AF require life-long anticoagulation therapy to prevent cardioembolic stroke and systemic embolism. The CHA2DS2-VASc risk score is a validated and widely used stratification tool to predict thromboembolic risk in atrial fibrillation patients and to identify patients who should benefit from anticoagulation therapy (LIP 2011; Camm, et al. (2012) Eur Heart J 2012; 33: 2719-2747); the accumulated evidence shows that CHA2DS2-VASc is at least as accurate as or possibly better than, scores such as CHADS2 in identifying patients who develop stroke and thromboembolism and definitively better at identifying ‘truly low-risk’ patients with AF. It is estimated that 85 to 90% of AF patients will require anticoagulation therapy.
In a meta-analysis comprising 6 trials which evaluated the effect of vitamin K antagonists (VKA) in reducing stroke and systemic embolism, a highly significant risk reduction in stroke incidence (relative risk reduction of 67% for stoke) was observed. All-cause mortality was significantly reduced (26%) by adjusted-dose VKA vs. control (Hart, Pearce, and Aguilar (2007) Ann Intern Med 2007; 146:857-867). An international normalized ratio (INR) target between 2 and 3 was associated with best benefit-risk ratio (Hylek et al (2003) N Engl J Med; 349:1019-1026) and universally adopted by international and national guidelines.
In recent years, new oral anticoagulants (NOAC) also referred to as direct oral anticoagulants (DOAC) have been approved and introduced to clinical practice. These drugs are at least as effective or even better than warfarin for reducing thrombo-embolic disease (Connolly, et al. (2009) N Engl J Med; 361:1139-51; Connolly, et al. (2011) N Engl J Med; 364:806-17; Patel, et al. (2011) N Engl J Med 2011; 365:883-91). NOAC were also associated with large reductions in the most devastating complications of warfarin namely hemorrhagic stroke and intracranial hemorrhage. Major bleeding events were similar or slightly lower than well conducted warfarin therapy. In addition NOAC are associated with a lower potential for drug-drug interaction than warfarin and could be used without routine monitoring; this is expected to ease their use in everyday medical practice.
Despite recent improvements, bleeding risk continues to be high with the use of anticoagulants. For instance, the annual incidence of major and clinically relevant non major bleeding was 14.9% and the annual incidence of major bleeding events was 3.6% in patients treated with rivaroxaban in the ROCKET study (Patel et al 2011). The annual incidence of major bleeding was >5% in patients at a high risk for bleeding defined as HAS Bled risk score ≥3 (Gallego, et al. (2012) Carc Arrhythm Electrophysiol.; 5:312-318). Major bleeding is a particularly relevant clinical outcome; for instance in the ROCKET study, once major bleeding has occurred, all-cause mortality rate was 20.4% in the rivaroxaban group and 26.1% in the warfarin group. Once major bleeding events have occurred stroke and systemic embolism occurred in 4.7% and 5.4% of patients in rivaroxaban and warfarin groups, respectively (Piccini, et al. (2014) Eur Heart J; 35:1873-80). Hospital stay, transfusion of blood products and resources utilization were also severely impacted by the occurrence of major bleeding. Bleeding risk is also a major reason for not receiving anticoagulants in eligible patients. In the Euro Heart Survey on Atrial Fibrillation comprising data from 182 hospitals in 35 countries and 5333 ambulant and hospitalized AF patients, only 67% of eligible patients received oral anticoagulant at discharge (Nieuwlaat, et al (2005) Eur Heart J; 26, 2422-2434). A high unmet medical need therefore exists for a safer therapy which can reduce AF thromboembolic complications such as stroke, systemic embolism, cognitive decline and mortality with comparable efficacy as existing therapy but with a lower bleeding liability.
Factor XI (FXI) holds important roles in both intrinsic and extrinsic coagulation pathways and in bridging the initiation and amplification phases of plasmatic hemostasis (Gailani and Renné (2007) Arterioscler Thromb Vasc Biol; 27(12):2507-13). Both Factor XII and thrombin can activate FXI, resulting in a sustained thrombin generation and fibrinolysis inhibition. FXI plays a minor role in normal hemostasis in a high tissue factor environment “after vessel injury” whereas it appears to play a key role in thrombosis. Severe FXI deficiency is associated with a lower incidence of ischemic stroke and venous thromboembolic events (Salomon et al (2008) Blood; 111(8):4113-7; Salomon et al (2011) Thromb Haemost; 105(2):269-73). Furthermore, in a population-based study, a survival advantage of severe FXI deficiency was evoked as a result of a lower incidence of thromboembolic events (Duga and Salomon, (2013) Semin Thromb Hemost; 39(6):621-31). Bleeding manifestations in subjects with severe FXI deficiency are infrequent, usually mild, injury-related, and affect preferably tissues with increased fibrinolytic activity such as the oral mucosa, nasal mucosa and urinary tract (Bolton-Maggs, (2000) Haemophilia; 6 Suppl 1:100-9). Bleeding in vital organs is extremely rare or not existing.
Accordingly, as part of efforts to lower bleeding liability, there is also a high unmet medical need for specific, reversal agents for anticoagulant therapies, for example, in circumstances when reversal of the anticoagulant effects of a therapy is needed for emergency surgery/urgent procedures and in life-threatening or uncontrolled bleeding.